The geological concentration and trapping of metal values in sediments by desert evaporation processes results in a particular class of mineral deposits and the processes of recovery and assay of these sediments must be tailored to their geochemistry.
Some such sedimentary systems of commercial interest are those of certain of southeastern California playas. The deposits are of commercial interest primarily because of the presence of economic amounts of PMs and PGEs although a large suite of other unusual elements is also present.
The Amargosa River drainage system of Inyo County, Calif. is a good example. The region constitutes an intensely evaporative regime with challenging metallurgic features. In the past few million years, relatively high enrichment of PM-PGE values in this evaporative trap has been effected by the erosion of Basin and Range Age (Cenozoic) felsic ash volcanos, and the washing of green dioritic ash deposits of the same age directly into the playa, accompanied by the evaporation of late magmatic geothermal solutions.
The geochemistry of several of the dry playa lakebeds of the area also is characterized by zeolization of volcanic ashes in the alkali playas, the transport of PM-PGE ions by creosote and other plant oil ligands in upper groundwaters, and also by alkaline thiosulfate anions, in deoxygenated lower-level groundwaters. Evaporative stranding has resulted in the accumulation of Be, B and Li (as well as Na, K, Ca and Mg) as carbonates and jarosites. Jarosites are gypsum-like minerals believed to be derivative of the desert oxidation of original sulphides--commercially viable borax is available in the area. At deeper (deoxygenated) levels the sulfate is re-reduced bacterially to thiosulfate. Finally, re-oxidation of PM-PGE thiosulfate complexes to jarosites occurs near the evaporative surface.
The PM-PGEs are believed to be present as oxidized salts in which the PM-PGE are in valence states above zero. In these oxidized valence states, the PM-PGE end up as minor lattice substituents in microscopic insoluble jarosite-type minerals. The jarositic minerals are either wholly or partly absorbed in the microscopic zeolitic pores of the clinoptilolite type clay zeolites present in this type of playa bentonite.
One of the chief chemical trapping mechanisms is believed to be upward wicking of soluble PM-PGE thiosulfate complexes followed by their reprecipitation as insoluble jarosites as the thiosulfate (S.sub.2 O.sub.3 .dbd.) ion re-oxidizes to the sulphate (SO.sub.4 .dbd.) ion above the water table. Another such trapping mechanism is believed to be alkali saponification, in the lakebed, of PM-PGE-creosote-oil ligand complexes which results in the PM-PGEs being dropped as insoluble inorganic salts in the same zone. Certain salts, such as Ag.sub.2 O, are soluble and continue on down drainage.
Unlike the mineralogy of other ore bodies, such as primary sulfide deposits, the mineralogy of these sediments is such that significant amounts of radioactive, rare earth, heavy alkali and heavy metalloid elements are caught in the sedimentary trap as well as the PM-PGE values. Finally, "chromatographic-type" depletion of many of the light transition metals (largely insoluble in thiosulfates) and base metals occurs in transport enroute from source to lakebed. In addition, the base metals are captured as sulfides in deeper levels of the lakebed. All of these factors have contributed to the anomalous nature of the upper playa-level geochemistry.
Various characteristics of the geochemistry of these types of playa evaporite sediments ensure that the PM-PGE values have evaded recovery to this point, or even detection, by existing "standard" methods used by the mining industry. This evasion of recovery either has been complete or was prohibitive of commercially significant processing.
Cyanide chemistry fails because of the insolubility of the host jarosites, and because the destruction of the CN ion is catalysed by the presence of rhodium salts in the sediments.
Aqua regia chemistry also fails because of the insolubility of the host jarosites and because of absorption of the zeolites. In this medium the rhodium is also a catalyst for the destruction of the NO.sub.3 ion.
Insolubility of the jarosites also precludes the use of mercury amalgamation as does the As, Sb and Te fouling of mercury. In addition, the extreme environmental toxicity of the mercury prohibits any commercial application of such technology.
Ammonium nitrate-sulfuric acid treatment was found to work rather poorly due to high acid consumption, as well as to the rhodium catalysis of the destruction of NH.sub.3 and to the insolubility of the jarosite in acid solution.
Oxidative leaches of the insoluble, already oxidized mixture of PM-PGE salts, were found to be generally ineffective. Gravity methods failed due to the minute particle size, plus the lack of an appreciable density contrast (no discrete PM-PGE-only minerals) and the absence of the native metal state.